As known by prior arts, in addition to the means of providing resonators in an intake path to muffle the noise of a specified range of frequency calculated on the basis of the resonance theory of Helmholtz and others, various types intake ducts have been employed for reducing the intake noise generated in an intake system at the time of taking air from outside into the internal combustion engine such as a gas engine.
According to Patent Document 1, an intake duct has a duct wall, all or a part of which is formed of a porous material having a vent function, and a covering component fitted outside of the duct wall formed of the porous material so as to cover the all or the part of the intake duct with an air layer formed between the duct wall and the covering component.
The intake duct of Patent Document 1 having the above structure allows vibration involved in pressure pulsation in the duct to pass through the duct wall formed of the porous material so as to be let out through the air of the air layer, having the full effect in suppressing the noise generation due to the pressure pulsation in the duct. Further, the porous material member is covered with the component other than the intake duct, which prevents dust and others drifting in the engine room from sticking to the porous material member, lowering the vent function and at the same time prevents water from coming through the porous material member into the intake dust.
According to Patent Document 2, an intake duct is provided with a porous sheet member having a joining portion, with its outer edge portion impregnated with a resin, integrally joined to the intake duct so as to cover an opening formed in a part of the wall of the intake duct.
The intake duct of Patent Document 2 having the structure mentioned above is integrally comprised of a plurality of individual sections, which makes it easy to conduct maintenance operation in the interior of the intake duct by only removing any of the individual sections. In addition, if any part in the individual sections is broken, only the individual section including the broken part has to be changed without changing the intake duct as a whole. Further, the intake duct of Patent Document 2 is manufactured by a method other than blow molding and hence, its components have each a desired thickness even in their differently complicated shapes, which gives a sufficient strength to the structure of the intake duct. Furthermore, the individual sections of the intake duct and the porous sheet member are joined together by the joining portion, which enhances adhesion between them, ensuring reliability of the intake duct as a whole.
[Patent Document 1] Unexamined disclosure No. 2003-343373
[Patent Document 2] Unexamined disclosure No. 2002-106431